Light- and electron-microscopic techniques are being used to examine developmental changes in synaptic and neuronal morphology in the rabbit striate cortex. Selected laminae are being examined in close comparison with already-gathered physiological data. Our attempt is to define the synaptic arrangements underlying receptive-field responses, and to note fine structural changes in the synaptic junction region. A second major project involves neonatal removal of one or more axonal inputs to the superior colliculus. Several months later, a remaining input to the superior colliculus is examined, to see if it has filled vacated laminae in the colliculus. Both light- and electron-microscopic techniques are employed.